1. Field of the Invention
This invention relates to a method for controlling electric resistance of a compound-type resistor such as, for example, a metallic-oxide-type resistor in the form of a film provided on a electrically insulated base.
2. Description of the Prior Art
FIG. 3 is a schematic illustration showing a conventional method for controlling electric resistance of a compound-type resistor described in a magazine entitled "Electronic Engineering", volume 25, No. 14, pages 32 and 33, issued in 1983. In this figure, reference numeral 1 designates a compound-type resistor material (hereinafter referred to as resistor material) such as, for example, a metallic-compound-type electric resistor material whose electric resistance is adapted to be controlled in an appropriate manner. Connected with the resistor material 1 are a pair of electrically conductive bases 2. A pair of probes 3 have their pointed tip ends in contact with the surfaces of the respective bases 2. The probes 3 are electrically connected with a controller 4 which is adapted to meter electric resistance of the resistor material 1 so as to control "on" and "off" switching as well as scanning of a laser beam 5 which is, for example, a continuously oscillated laser beam or a pulsed laser beam having short interval pulses.
With the above-described arrangement, the laser beam 5 issued from a laser source (not shown) such as a laser oscillator is condensed by means of a known appropriate optical system so as to be focused upon the resistor material 1 which is to have electric resistance controlled. Then, the pair of probes 3 are placed in contact with the surfaces of the electrically conductive bases 2, which are connected to the opposite ends of the resistor material 1, respectively, for measuring electrical resistance of the resistor material 1, and at the same time, the laser beam 5 is irradiated from the laser source (not shown) onto the resistor material 1 so that a part of the resistor material 1 is instantaneously vaporized and thus cut away, thereby increasing electric resistance thereof. As a result, by cutting away the resistor material 1 to provide a desired value of electric resistance by means of the laser beam 5, it is possible to obtain a resistor having the required resistance value.
With the above-mentioned conventional method for controlling electric resistance of a compound-type resistor material, however, electric resistance of a resistor can only be controlled so as to increase resistance. Accordingly, it is necessary to set the electric resistance of a resistor, prior to control of its electric resistance, at a value lower than the desired value. To this end, the area of the resistor marterial 1 must be set at a larger value because the thickness of the compound-type resistor material such as a metallic-oxide-type resistor in the form of a thick film, which is usually produced by screen printing technique, can not be increased to a value greater than about 10 .mu.m. For this reason, the area of a base board to be occupied by the resistor mounted thereon necessarily increases, thus posing a problem in that it is difficult or impossible to miniaturize a hybrid IC mounting thereon such resistors as well as to increase density or efficiency in arrangement thereof.
Also, there is another problem in that a cutting groove, formed in the resistor material 1 by irradiation thereon of the laser beam 5, is liable to be subject to generation of micro cracks at the peripheral edge thereof so that with the passage of time such a compound-type resistor undergoes changes in electric resistance thereof.
Moreover, in cases where the resistor material 1 is cut to an excessive extent by means of the laser beam 5, it is impossible to remedy or revise such an excessive increase in the resistance value of the resistor material 1 due to the fact that the electric resistance of the resistor material 1 can by no means be decreased. In fact, if the cutting speed, at which the resistor material 1 such as a metallic-oxide-type resistor material in the form of a thick film produced by screen printing is cut by a laser beam, is increased beyond a certain extent, there will be a time lag between measurement of the resistance value of the resistor material 1 and scanning of the laser beam 5 so that the resistor material is cut excessively, rendering the resistance value thereof higher than a desired value. For this reason, the cutting speed of the laser beam 5 can not be increased very much and it is difficult to obtain the objective resistance value.